(1) Field of the Invention
The present invention generally relates to integrated circuits. More specifically, the present invention relates to a method for forming air gaps in an interconnect system of an integrated circuit.
(2) Description of the Related Art
Modern integrated circuits are generally made up of millions of active and passive devices such as transistors, capacitors, and resistors disposed onto a silicon wafer. These devices are initially isolated from one another, but are later interconnected together by an interconnect system to form functional circuits. The quality of the interconnection of these devices drastically affects the performance and reliability of the fabricated integrated circuit.
An interconnect system typically includes metal lines, spaced apart from each other, which interconnect the various active and passive devices found in a silicon wafer onto which the interconnect system has been deposited and fabricated. Insulating dielectric layers are deposited between the metal lines for isolating the metal lines from one another. Inherent in the structure of the interconnect system is a capacitance associated with the metal lines spaced apart from each other. Decreasing this capacitance is desirable as several advantages can be achieved therefrom, such as reduced RC delay, reduced power dissipation, and reduced cross-talk between the metal lines. Since the capacitance is inversely proportional with the distance between the metal lines, one way to reduce the capacitance between two lines would be to increase the space between the lines. However, this option is not desirable because of the limitations imposed by packing density.
One way to reduce the capacitance between two lines of an interconnect system is to reduce the dielectric constant of the dielectric material deposited between the metal lines, as the capacitance is directly proportional to the dielectric constant of the dielectric material between the metal lines. One dielectric material typically used to isolate metal lines from each other is silicon dioxide (SiO.sub.2). Silicon dioxide is a thermally and chemically stable material. The dielectric constant of SiO.sub.2 is approximately 4. The dielectric constant is based on a scale where 1.0 represents the dielectric constant of a vacuum. Various materials exhibit dielectric constants from approximately 1.0 to values in the hundreds.
The dielectric constant of SiO.sub.2 is, however, high. Recent attempts have been made to use organic polymers that have low-densities and dielectric constants lower than those of silicon dioxide to replace silicon dioxide as a dielectric material thereby reducing the capacitance between the metal lines. Organic polymers are chemical compounds made up of repeating units that have high coefficient of thermal expansion (CTE) (typically around 60 ppm/.degree. C.), which allows them to expand and shrink more rapidly than materials having lower CTEs. To reduce the dielectric constant of the organic polymer, recent attempts have focused on creating air gaps within the organic polymer. Since air has a dielectric constant of about 1, the use of air gaps between the metal lines reduces the dielectric constant of the polymer between the interconnect lines thereby reducing the overall interconnect system capacitance. However, conventional methods of forming air gaps, typically by non-conformal interlayer dielectric (ILD) deposition, may leave the voids vulnerable to chemical processes, such as etching or chemical polishing during fabrication of the interconnect system. Voids created by conventional methods are formed when the surfaces of two opposite sidewalls of a low-dielectric constant material meet centrally to dose and fill a trench defined by two metal lines. Sometimes, however, reactant gas may have access through the seams and be trapped in the voids. Also, the voids may be left partially open during a chemical process step such that foreign materials may enter the voids. Trapped solvents create reliability problems during a high temperature process, such as chemical vapor deposition (CVD), since these solvents tend to evaporate thereby causing the formed void to explode.
What is needed is an improved method for forming air gaps in an interconnect system that avoids the reliability problems associated with conventional methods of forming air gaps.